This invention relates to a filter composite useful for filtering particulate matter, including radioactive particles, while at the same time venting gaseous species.
The art provides many types of filters for gases and particulate materials. See, for example, U.S. Pat. Nos. 4,234,326; 4,157,248; 4,133,651; 4,160,059; 3,944,403; and 4,314,828. Such devices are based upon powdered, granular, or fibrous carbon, and sometimes combinations thereof (U.S. Pat. No. 3,944,403), which remove, filter or capture gaseous or particulate substances. More often than not, the prior art devices are designed for gas filtration rather than gas absorption. The gas filters of the art, while fairly effective in the applications for which they were specifically designed, do not approach the very high efficiency achieved by the banks of air filters known as high efficiency particulate air (HEPA) filters. These latter filters, which are used in filtering air contaminated with highly radioactive particulate materials, retain up to 99.97 percent of 0.3 .mu.m particles, as measured by the dioctylphthalate (DOP) smoke test (Military Specification F0051079D). A simple filter capable of such efficiency could find ready acceptance in the nuclear waste storage field, especially on containers in which low level gas-generating radioactive waste is kept.
Another technical area where better filters could be used to advantage is in the filtration of air or other gas streams where high gas pressures demand high compressive strength or where high process temperatures prevail, e.g. up to 2000.degree. C. Under such conditions, the filters of the art are somewhat inadequate.
There is a need, therefore, for improved gas filters of high compressive strength that are resistant to high temperatures, chemicals, vibrations, and shock. There is also a need for a high efficiency filter that can be used for venting low radioactive level gas-generating nuclear waste when election is made to store such material in vented contains.